1. Field of the Invention
The present invention relates to a virtual machine system for controlling multiple virtual machines, in particular, to a system for handling an input/output (I/O) interrupt generated by an I/O device during the operation of a virtual machine.
2. Description of the Related Art
In a computer system, in order to realize the effective use of system resources, it is well known to construct a virtual machine system comprising one or more virtual machines realized by an existing real machine comprising by hardware.
In the virtual machine system, each virtual machine is controlled by a virtual machine control program (hereinafter called the "VM monitor") of a real machine system. Namely, control of a virtual machine, to which the control right over a CPU (central processing unit) has been transferred by the VM monitor, is carried out by the CPU in the real machine.
System control of the virtual machine is carried out by an operating system of the virtual machine. However because such control affects another virtual machines, the efficiency of the entire real machine system is decreased by interposition of the VM monitor, etc.
For example, when the virtual machine instructs an I/O start for an I/O device, if an I/O operation resulting from the I/O start is completed by the I/O device, the I/O device generates an I/O interrupt for the CPU, realized by the interposition of the VM monitor. Namely, when the I/O interrupt is generated by the I/O device during an operation of the virtual machine, if the I/O interrupt is enabled, an interrupt generating process for the real machine is carried out to transfer the control right over the CPU to the VM monitor of the real machine. The VM monitor carries out an interrupt handling routine to analyze the contents of the I/O interrupt, etc. Then, if the virtual machine, which has generated the I/O instruction to cause the I/O interrupt, is in an interrupt enabled state, the VM monitor simulates the generation of the I/O interrupt for the virtual machine to transfer the control right over the CPU to that virtual machine, whereby an interrupt handling routine is carried out in that virtual machine.
As described above, the I/O interrupt in the virtual machine system must be performed with the interposition of the VM monitor, so that time is necessary for the execution of the VM monitor in addition to time for processing a primary task such as interrupt processing. As a result, overhead for controlling the virtual machine is increased, which reduces the processing power of the virtual machine system.